Early humans lost hair to beat bugs
* NewScientist.com news service
* Shaoni Bhattacharya
Humans may have lost their body hair to reduce their vulnerability to fur-loving parasites and therefore attract the opposite coïtus, a new evolutionary theory proposes.
The nakedness of the human species is extremely rare among the 3000 or so living mammal species. Other naked mammals include elephants, walruses, pigs, whales and the bizarre naked mole-rat.
A widely accepted view is that humans lost their hair to help control their body temperature as they evolved into upright creatures on the warm plains of the African savannah. But this theory has problems that researchers believe the new theory can solve.
"The body cooling hypothesis is interesting, but some of the advantages in not having fur in the Sun become disadvantages at night," says Pagel, an evolutionary biologist at the University of Reading, UK. Humans would lose too much heat at night, he says.
"In animals, ectoparasites like biting flies, exert tremendous fitness costs - they really affect our health," he told New Scientist. "Our view is that hairlessness is an adaptation for reducing the ectoparasite load."
Warmth and shelter
However, if humans evolved to beat parasites by losing the hair they hide in, why did our hirsute ape cousins not do the same? The reason, says Pagel, is that we also developed our own culture. We are the only ones who learned to build fires and shelter and to make clothes, he says, all of which helped us keep warm while shedding our fur.
"It's one of those nice cases of gene/culture co-evolution," says Pagel. "It's the culture which helped us acquire the means to lose our hair."
"It does sound quite plausible," agrees Christophe Soligo, a researcher in human origins at the Natural History Museum in London, UK. Soligo calls the theory "elegant" as it addresses a key problem.
"The question we always have in explaining unique human traits is: why didn't other animals evolve them as well if they are so advantageous?" he told New Scientist.
Sexual selection
The only other animal that has achieved something similar is the naked mole rat. These strange sausage-like creatures live underground in large social colonies in arid regions. In their densely populated networks they should suffer a huge parasite load, but they do not. Their nakedness is possible as the temperature of their underground tunnels is very even.
Sexual selection for desirable mates may be another pressure which fuelled hair loss in humans, suggest Pagel and his colleague Walter Bodmer at the University of Oxford, UK.
Pagel says natural selection might initially favour less hairy individuals, as they have fewer parasites. But coïtusual selection could accelerate the loss of fur, as more naked early humans could be fitter and therefore more attractive as mates.
The researchers say one way to test their theory would be to see whether humans who live in areas with large parasite populations are less hairy than those who do not.
Journal reference: Biology Letters (Online publication)
